The invention relates to a method of preparing an aluminium oxide-containing quartz glass activated by bivalent europium in which a glass matrix oxide mainly comprises SiO.sub.2 and furthermore per mol of SiO.sub.2 up to at most 0.15 mol of MO, in which MO represents at least one of the oxides Al.sub.2 O.sub.3, B.sub.2 O.sub.3, P.sub.2 O.sub.5, ZrO.sub.2, Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Lu.sub.2 O.sub.3, the alkali metal oxides and the alkaline earth metal oxides.
A luminescent quartz glass activated by bivalent europium is known from a publication in J. Electrochem. Soc., Vol. 117, no. 5 (1970), 708-711. In this article the starting material is a glass having a very high SiO.sub.2 content (at least 96% by weight) which is activated by impregnating the glass with an aqueous solution of a salt of trivalent europium, notably Eu(NO.sub.3).sub.3 whereafter the glass is heated at a high temperature. This heat treatment is performed in a reducing atmosphere in order to bring the europium into the desired bivalent state and to incorporate it into the glass matrix in that state. The publication also states that luminescence is only obtained with these glasses if trivalent aluminium is also present in the glass matrix. Therefore the starting material is not only impregnated with a solution of a europium salt but also with an aqueous solution of an aluminium compound, notably Al(NO.sub.3).sub.3.
A drawback of the known method is that in many cases materials are obtained with a comparatively low luminous flux which are not suitable for practical uses. In fact, it has been found that the glasses, particularly at Eu concentrations above the relatively low value of 0.5 mol.%, often exhibit a yellow colour which implies that the reduction of the europium is not complete and that the degree of reduction is not satisfactorily reproducible.
The object of the invention is to provide a method of preparing luminescent quartz glasses activated by bivalent europium, which exhibit high luminescence efficiencies in a reproducible manner.